Answer / javad ahamad

both e/f relays are different. consider an alternator, its
neutral is earthed through a
resistance inorder to limit earth fault current .then we
cant protect the 100% winding from
differential e/f relay only.because if an e/f occurs at
very near to neutral point the driving
current to operate relay will be very small. there for it
cant be sensed by this method,
we can protect up to 80%(approximate value.depent upon
resistance)only.at this situation
we r providing a restrcted e/f relay to protect the
remaining winding ie 20%.this relay
have very low current setting.by the combination of these
two we can protect 100% winding.

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